Production of hard metal powders



United States Patent 2 Claims. cl. 241-22 The present invention relates to the production of hard metal powders. By hard metal when used herein and in the claims hereof I mean a carbide of a metal of the 'IVth, Vth or VIth groups of the periodic table, or a mixture of such carbides in a binder or matrix of at least one of the elements of the iron group, viz., iron, nickel and cobalt, usually in quantities of from 4% to 30% by weight of the hard metal. Cobalt is frequently employed as the binder or matrix.

It is known that workpieces consisting for instance of steel, alloyed steel, nickel alloys or the like can be provided with an abrasion-resistant surface by applying a hard metal layer of suitable thickness to the surface of such parts. This can be done by fusing suitably composed easily melting hard metal alloys on to the same. Alternatively such layers can be formed by the known powder spraying technique. In the latter process the method of procedure in principle consists in passing the powder through a high temperature zone for converting the powder into the molten state.

The molten powder is then sprayed on to the surface of the parts which are thus to be armoured and the surface itself may at the same time or previously be superficially melted with the aid of a supplementary electric arc. In order to permit these alloys to be used in spraying devices they must first be reduced in size, i.e., pulverised, and after pulverisation they may be mixed with other auxiliary alloys. The purpose of these auxiliary alloys is to lower the melting point of the pulverised mixture.

A known alloy which is suitable for this purpose may have the following composition Percent Nickel 65-75 Chromium 13-20 Boron 2.8-4.5 Carbon 0.5-1.0 Silicon 3.5-4.5 Iron 3.0-4.0

If sufficiently high temperatures are applied when spraying the powder or powder mixture, for instance as is the case in the known plasma spraying process, even pulverised hard carbide-binder-metal alloys (i.e., so-called hard metals) or even carbides without the binder phase can be sprayed or fused on to a part, and an additional auxiliary alloy based for instance on nickel-chromium-boron-silicon can be dispensed with. In any case, the problem still remains of pulverising the hard carbides or hard carbidemetal-binder alloys before they are sprayed by means of one of the said spraying techniques.

The size reduction of hard metals is a matter of considerable difiiculty because of the known mechanical properties of these materials. It is the object of the present invention to propose a method which permits hard metals to be size-reduced by mechanical means. According to the invention this is done by using conventional boroncontaining hard metals based on hard carbides and binders having a boron content of 0.05%-2%, preferably 0.1 to

ice

0.8% for the production of such hard metal powders by the size reduction of pieces of hard metal. In these materials the boron must be present in elementary form or as a boride. The simplest method for the production of the alloy is to add the boron in suitable quantities in the form of a boron key alloy, such as ferroboron, nickel-boron or cobalt-boron.

The specified boron content reduces the ultimate flexural strength of the hard metal to such an extent that mechanical disintegration, for instance in jaw crushers', ball mills or the like, becomes possible.

The following examples are intended to illustrate the effect of the boron addition.

alloy containing 0.1% boron had an ultimate flexural strength of only 117 kg./ sq. mm.

Example 2 Percent Tungsten carbide Cobalt metal e 20 The following table gives the ultimate flexural strength at room temperature Kg./ sq. mm. Without a boron addition 250-300 With 0.05% boron 150-180 With 0.1% boron -150 With 0.5% boron 60-100 Example 3 Percent Tungsten carbide 93 Cobalt metal 7 Ultimate flexural strength:

Kg./ sq. mm. Without boron addition 230-280 With 0.5% boron 30-80 Example 4 Percent Tungsten carbide 89 Cobalt metal 11 Ultimate flexural strength at room temperature:

Kg./ sq. mm. Without boron 246-276 With 0.5 boron 67-77 Example 5 Percent Tungsten carbide 86 Cobalt metal 14 Ultimate flexural strength at room temperature:v

-Kg./sq. mm. Without boron 283-3-17 With 0.5 boron 58-77 3 Example 6 Percent Tungsten carbide 80 Cobalt metal 20 Ultimate flexural strength at room temperature:

Kg./ sq. mm. Without boron 245-278 With 0.5% boron 88-101 Apart from the advantage that these boron-containing hard metals can be readily size reduced by mechanical means, their application according to the invention has the further advantage that the embrittlement due to the boron perceptibly raises the welding strength of the layer sintered or sprayed on to the metal surfaces.

Finally the following should be noted:

In the plasma spraying technique-as well as in some other spraying methods in which the sprayed hard metal powder is brought to melting point-the powder passes through a high temperature zone. This causes carbide decomposition, for instance a decomposition of the tungsten carbide WC to carbon and the carbide W C. The carbon which is liberated in this reaction is precipitated in the sprayed layer in the form of graphite and causes .pores to appear in parts of the layer. Experiments have disclosed that when using the boron-containing material according to the invention the decomposition of carbides is largely suppressed. It was also observed that the oxygen which is always contained in the initial powder partly reacts with the boron and partly with the free carbon present when boron-containing hard metal is used. In other words, this means that the reaction C+O -+2CO is promoted. Consequently the oxygen content in the sintered layer is lowered and at the same time the consumption of carbon leads to the formation of some socalled double carbides which, as known, are abrasionresistant phases.

The following Example 7 contains a table of the carbon and oxygen turnover which occurs in the production of a tungsten-carbide-titanium-carbide-tantalum-carbide metal.

Example 7 The primary hard metal used contained Percent Tungsten carbide 50 Titanium carbide 35 Tantalum carbide 7 Cobalt metal 8 The sintered final hard metal was found to contain the following contents of carbon and oxygen:

Percent Percent C 0,

Without boron 9. 62 0. 62 With 0.05% boron from O0/B 9. 45 0. 11 With 0.1% boron from (Jo/BM". 0. 35 0. 09 With 0.5% boron from C0/B 8.98 0. 04

Another advantage of using boron-containing hard metals for the specified purpose according to the invention resides in the fact that hard metal alloyed with boron bonds better and more quickly to the base material when it sinters after having been sprayed. This is probably due to the fact that diifusion-inhibiting oxides are absent or that they are removed during the spraying operation. If, as above indicated, a chromium-nickel-boron-silicon auxiliary alloy is used, then the presence of boron in the hard metal component also promotes the generation of a diifusion bond between this component and the binder metal and/or the base metal. These advantages are of such importance because all diffusion processes must proceed within the very short time available for welding.

What I claim is:

1. The method of producing a hard metal powder from a hard carbide metal which comprises dispersing boron in the matrix for the carbide metal in a quantity of from 0.05% to 2% of the carbide metal and mechanically disintegrating the carbide metal to form a powder suitable for spraying or fusing to form a coating on a workpiece.

2. The method of producing a hard metal powder from .a hard carbide metal which comprises dispersing boron in the matrix for the carbide metal in a quantity of from 0.1% to 0.8% of the carbide metal and mechanically disintegrating the carbide metal to form a powder suitable for spraying or fusing to form a coating on a workpiece.

References Cited by the Examiner UNITED STATES PATENTS 1,739,068 12/29 Harris 241-15 2,394,052 2/46 Hall et a1 241-16 2,726,045 12/55 Hinerfeld 241-16 2,936,229 5/60 Shepard 0.5 3,155,491 11/64 Hoppin et al 750.5

OTHER REFERENCES Treatise on Powder Metallurgy, Goetzel, vol. II, copyright 1950 (pp. 744-748). (Copy in Sci. Lib.)

ROBERT C. RIORDON, Primary Examiner.

I. SPENCER OVERHOLSER, Examiner. 

1. THE METHOD OF PRODUCING A HARD METAL POWDER FROM A HARD CARBIDE METAL WHICH COMPRISES DISPERSING BORON IN THE MATRIX FOR THE CARBIDE METAL IN A QUANTITY OF FROM 0.05% TO 2% OF THE CARBIDE METAL AND MECHANICALLY DISINTEGRATING THE CARBIDE METAL TO FORM A POWDER SUITABLE FOR SPRAYING OR FUSING TO FORM A COATING ON A WORKPIECE. 